1. Field of the Invention
The present invention relates to a control device which controls a controlled variable of a multiphase electric rotating machine by controlling a power inverting circuit having a plurality of switching elements and freewheel diodes connected with input and output terminals of the respective switching elements such that each switching element electrically connects or disconnects a positive electrode or a negative electrode of a direct current power source to or from one of terminals of the machine.
2. Description of Related Art
A control device for a three-phase motor has been used. For example, this device performs pulse width modulation (PWM) control, based on triangular wave comparison, for the motor to control three phase currents, actually flowing through respective phase windings of the motor, to an instructed value under the feed-back control. In this PWM control, an instructed value of phase voltages to be applied from an inverter to the respective phase windings of the motor is calculated, and switching elements of the inverter are operated based on the difference between the level of a carrier signal shaped in a triangular wave and the instructed value.
However, when this PWM control is performed in an excessive modulation region in which the instructed voltage value is higher than an input voltage of the inverter, levels of higher harmonic waves contained in the output voltage of the inverter increase. In this case, the higher harmonic waves undesirably influence the response characteristics of the phase currents flowing through the motor. This problem is arisen because it is assumed in the design of a current control system that the output voltage of the inverter can be always set at the instructed value.
To avoid this problem, Published Japanese Patent First Publication No. 2008-228419 proposes a model prediction control performed in a control device. In this control, a plurality of operating states of an inverter applying a voltage to a three-phase motor are preset, a current expected to flow through the motor is predicted from the output voltage of the inverter, on the assumption that the inverter is set in one operating state, for each of the operating states, and the inverter is actually set in one operating state which minimizes the difference between the predicted current and an instructed current. Therefore, because the inverter is operated so as to optimize the variation of the predicted current, the problem described above can be avoided to a certain degree.
Japanese Patent Specification No. 393554 also proposes a control device for a three-phase motor.
However, even when the current is predicted in the model prediction control, the predicted current includes an error. Therefore, the current actually flowing through the motor is still shifted from the instructed current, and the difference between the actual current and the instructed current sometimes becomes large. The error occurring in the predicted current is derived from an error arisen in the model prediction control.
This error is not only generated in the current prediction, but also generated in the prediction of a controlled variable (e.g., torque or magnetic field) of the electric rotating machine.